Elvis,
Basically, the necks are too soft. You can feel it when seating bullets, and accuracy usually ain't great--but if accuracy improves after the first firing, then you're pretty sure!
In the instance I cited, a rifle that usually groups 5 shots well inside an inch started shooting 3-shot groups over an inch. After the first firing, things got back to normal.
As has been discussed earlier, heating brass to 800 degrees F. for a few seconds does the job. Heating it hotter and longer can make it too soft--which is often what happens when heating it "red hot," however that is defined or perceived.
